Smart cards may transmit data between a CPU core and peripherals via a bus system. The data must be protected against attacks such as spying. The data may for these purposes, be provided with a dynamic mask. A successful external attack on a peripheral may make it possible to manipulate the peripheral, for example through error induction, so that the peripheral may accept certain data not intended for this peripheral. Furthermore, the dynamic mask may be removed from the data. Therefore, it may be possible for an attacker, during the manipulation to watch the data. Up till now, no known practical methods of preventing this are known.